\documentclass{article}
\usepackage{threeparttable}
\usepackage{graphicx} 
\usepackage{booktabs}
\begin{document}
\begin{table}[h]
\centering
\resizebox{1\width}{!}{ 
\begin{threeparttable}
\caption{Summary Statistics for the Chinese Exporters to the US}
\begin{tabular}{lcccc} \hline\hline
Scope                       &     1        &   2         &    3-4   &  5+      \\
\hline
Exporter Share with Scope   & 41      & 20     & 19  &  20 \\
Mean Total Firm sales       & 907      & 1826     & 3141  & 10185  \\
Mean Top Ranked Product         & 907      & 1679     & 2689  &  7184 \\
Mean Bottom Ranked Product  & 907      & 147     & 45  & 8  \\
\hline\hline
\end{tabular}
\begin{tablenotes}
\small
\item  \noindent  \footnotesize{The table splits the sample of exporters from China to the US according to the number of products that they exported in 2003  into exporters with 1,2,3-4, and 5 or more products. For each group it reports the share of exporters, mean total exporter sales and mean sales of the best and least-selling products. Sales are in thousands of USD.}
\end{tablenotes}
\end{threeparttable}
}
\end{table}
\end{document}
